1. Field of the Invention
This invention concerns an ATM(asynchronous transmission mode) switch in an ATM network, and more particularly concerns a method for path control during signal line impairment in the ATM switch.
2. Description of the Related Art
The use of ATM network today is being expanded rapidly by LAN/WAN and internet applications, and it is now predicted that its use will spread to data switching in VOD (video on demand) services.
FIG. 31 is a diagram of an example configuration in an ATM network. According to FIG. 31, a preexisting telephone network, the internet, and a VOD network are connected via ATM networks.
ATM switching services are now shifting from the PVC (permanent virtual channel) services currently popular for dedicated line services to SVC (switched virtual channel) services that operate on an on-demand basis. SVC services include TPCC (third party call control) services that control signal lines by proxy, so that subscribers who do not have signal lines for transmitting call-establishing and other signal cells can avail themselves of SVC services. This TPCC service is able to control the paths to multiple terminals with a single signal line, and is beginning to be used in services such as VOD.
FIG. 32 diagrams an example of a TPCC operating configuration. In FIG. 32, a subscriber device (terminal) A having no signal line uses a preexisting telephone network (cf. line (1) in the figure) to request to establish or release an SVC path (cf. line (3)) with a signal proxy server (terminal) C having a signal line. When the link between the subscriber device A and the signal proxy server C is established by a PVC-based path (cf. line (2)), the request to establish or release the SVC path (cf. line (3)) is conducted by the PVC path. The signal proxy server (terminal) C that receives the request from the subscriber device A provides an SVC path (cf. line (3)) between the subscriber device A and terminal B (which is either another subscriber device or a VOD server), via an ATM switch D, using a signal line (cf. line (4)) that is for SVC performance.
A point-to-multipoint service that is a 1-to-n connection configuration is also a service wherewith one subscriber device can establish paths with a plurality of other subscribers, with a single signal line.
FIG. 33 is a diagram of an example point-to-multipoint (hereinafter P-MP) operating configuration. According to FIG. 33, multiple paths are established between one subscriber device and a plurality of other subscriber devices by means of the P-MP signal control functions, P-MP call control functions, and cell copy functions of an ATM switch.
Thus, from this time forward, the mainstream will increasingly become SVC connection services that are provided using signal procedures that can appropriately select and designate path (virtual channel) bands, quality, and traffic according to the data cell being transmitted.
With the prior art, however, in situations where SVC services are implemented with various operations expected thereof, as discussed above, when a signal line is impaired, all paths controlled by that signal are released, and the call (path) cannot be reestablished until that signal line is restored.
However, forcibly releasing established paths just because a signal line is impaired leads to a decline in service quality. It is foreseen, moreover, that among the highly diverse services that are now going to be using ATM switching will be those wherewith data transmission interruptions cannot be allowed.
In VOD services, for example, when paths are connected via ATM switch between a server providing data and multiple subscriber devices, should an impairment occur in a signal line between the server and the ATM switch, this impairment of a single signal line would unavoidably result in the release of all paths between the server and the subscribers and, hence, a decline in subscriber service.
Accordingly, an object of the present invention is to provide ATM switch wherewith, in an ATM network system, data transmission will not be interrupted even when an impairment occurs in a signal line.
In order to achieve this object, the ATM switch of the present invention comprises a detector that detects impairments in signal lines, and a controller that controls the preservation of paths that have been established by call control via a signal line when an impairment has been detected on a signal line by the detector.
When an impairment has occurred only on a signal line, it is possible to prevent interruptions in communications services by causing the data line paths to be preserved rather than released.
Preferably, the controller should send a information of the fact that the path has been preserved to the terminals or to another asynchronous transmission mode switch connected to the path, as noted earlier, via the path.
And, preferably, the controller should release the path, based on a path release request information sent via the path from either a terminal connected to the path or from the another asynchronous transmission mode switch.
And preferably, as soon as the signal line has been restored, the controller should send a signal line restart information via the path, to the terminals or the another asynchronous transmission mode switch.
Because call control cannot be performed using a signal line due to an impairment on the signal line, information needed for various kinds of call control are sent using a path that is the data line. At such times, the information sent over the path may be configured by OAM cells, for example. Other features and advantages of the present invention will become readily apparent from the following description when taken in conjunction with the accompanying drawings.